1. Field of the Invention
The present invention relates in general to a vacuum refining method which utilizes a ladel and an induction heating technique, and more particularly to such a vacuum ladle refining method suitable for refining a relatively small amount of molten metal, such as steel, special steel and stainless steel.
2. Discussion of the Prior Art
A conventional method for producing metals such as special steel includes a process of refining raw materials, such as a mass of molten metal obtained by melting a scrap in an arc furnace, or a mass of molten iron tapped from a blast furnace. While various methods have been proposed for the refining of the metals, these methods suffer from some problem as described below.
As one method using a fixed furnace, for example, there is known an AOD method for refining molten metal in air, with Ar gas and O.sub.2, gas blown from the bottom of the furnace through the molten metal to be refined. There is also known a GRAF method, according to which a furnace body of the furnace is tilted after refining of the molten metal by electrode heating, so that a tuyere of the furnace is positioned under the surface of the molten metal, and Ar gas is blown through the tuyere into the molten metal. However, these methods are unsatisfactory in degrees of deoxidation and dehydrogenation and in removal of nonmetallic impurities. In addition, these methods using fixed furnaces need to use another ladle for transferring the refined molten metal to a location of casting. Thus, the molten metal tends to be affected by the air during its transfer into this ladle. Further, if only a small amount of molten metal is to be refined, the use of such a fixed furnace results in considerable deterioration in refining capability, and makes the refining apparatus comparatively large-sized in contrast to the amount of the molten metal. Moreover, since the refining temperature is elevated to a considerably high degree, refractories used for the fixed furnace should be highly resistant to heat.
In view of the above drawbacks, there is proposed another refining method called LF method, according to which a mass of molten metal is poured into a ladle, and then refined within the ladle. This LF method is advantageous in that the molten metal can be easily transferred to a location of casting without being largely affected by the air, and in that the molten metal can be refined into an extreme low oxygen steel, for example, without taking account of unfavorable influences on the components of the metals. To improve the quality of metals, there is further proposed a so-called VLF method (vacuum ladle refining method) as one method of refining a mass of molten metal under vacuum, outside of the furnace. According to this VLF method, the ladle which contains the molten metal is fluid-tightly closed by a top lid, and heating electrodes are inserted through the top lid into the ladle, so that the molten metal is heated by an electric arc generated by the electrodes, and thus slag-refined. Then, the electrodes are removed, and the top lid is replaced by another lid which permits degassing of the molten metal under vacuum. In this method, it is difficult to maintain a suitable degree of vacuum, since the ladle cannot be kept under vacuum during arc-heating, and the molten metal cannot be heated during degassing thereof. Therefore, the above method does not provide a sufficiently high degree of degassing effect. Further, this method requires two kinds of top lids so as to perform heating and degassing operations separately, resulting in a rather complicated refining process and an increase in the cost for preparing the refining apparatus including the ladle and the top lids.
As an alternative to the VLF method, it is proposed to employ an induction heating technique instead of the above-described electrode arc heating technique. According to the method utilizing the induction heating technique, a coil is disposed around a ladle which contains a mass of molten metal, and an electric power having a given frequency is supplied to the coil so as to heat the molten metal in the ladle by induced electric current. At the same time, an upper opening of the ladle is fluid-tightly closed by a top lid, and the air is sucked from the ladel so that the interior of the ladle is kept under vacuum. Thus, this method permits heating of the molten metal and evacuation of the ladle to be effected concurrently. However, ambient atmosphere is undesirably introduced into the ladel through its wall made of porous refractories, whereby the degree of vacuum in the ladle is reduced. Thus, it is difficult to keep the interior of the ladle in a highly evacuated condition.